Device for the measuring and comparing of electrical impedances



May 25, 1937. J UNK I 2,081,690

DEVICE FOR THE MEASURING AND COMPARING OF ELECTRICAL IMPEDANCES Filed July 22,1933 2 Sheets-Sheet 1 .1. UNK

' May 25, 1937.

DEVICE FOR THE MEASURING AND COMPARING OF ELECTRICAL IMPEDANCES Filed July 22, 1953 2 Sheets-Shet? Irwenior: Jacob M U n17,

Patented May 25, 1937 2,081,690 nsvrcs roa 'rns MEASURING AND PATENT women COM PARING OF. ELECTRICAL IMPEDANCES Jacob Marlnus Unit, ElndhovemNetherlands, assignor, by meme assignments, to Radio Corporation of America, New

' poration of Delaware f York, N. Y., a corlipplication July 22, 1933, Serial No. 881,791

In Germany July 19. 1932 '5 Claims. 101. 175-183) My invention relates to devices for the measuring and comparing of electrical impedances and more particularly to devices and methods by means of which such measurements can be made 5- with a high degree of accuracyand the disturbing influences of stray or parasitic impedances can 1 be properly compensated for or is altogether elim-" The measuring and matching of impedances l9 with ahigh degree of accuracy has an increasing practical importance in-electrical engineering.

This isespecially true with regard to' radio apparatus and for instancein radio-receiving de-.

vices tuned by the adjustment of a single dial. the

i6 increasing requirements imposed upon the quality of such sets, especially'the requirement of high selectivity makes the accurate determination and l matching of the impedances of the circuits of suc but little skilled labor.

The most suitable method of measuring and comparing impedances is by the well-known Wheatstone bridge arrangement. However; in I 80 view of the small values-of the capacitiesjand impedances used in these circuits and of the highfrequencies at, whichthese, devices are used and thus the tests have to be made, considerable air ficulties are introduced. v

This is primarily due to the fact that inthee high. frequency tests the influence of'so-calied stray or parasitic impedauce'sandespecially of parasitic capacities occurring between the component parts of the device and betweensuch parts ments and distort the results.

The object of my invention is the elimination or neutralization g1 such tests of the parasitic 40 and the surroundings, vitally affect the measure- 45 A further object of my invention is to obtain 1 the above result by simple means andto provide for devices in which such measurements can be made in a quick manner and by substantially unskilled labor. Further objects of my invention will appear as the specification progresses.

- To neutralize the stray impedances in such devices, it has been already suggested to screen from each other the various component parts by a means of casings made of conductive material impedances and es ecially of parasitic capacities.

which are interconnected to each other and/or connected to ground.

In the known devices the bridge is screened from the surroundings .and from the high frequency generator supplying the measuring current, whereby as a rule one terminal of this generator is grounded. I

vWhen a high frequency dynamo is used its windings must be statically screened from each other. This entails considerable losses and requires the dynamo .to be greatly over dimensioned compared to the actual energy requirement of they bridge. As the generator represents a major portion of the cost of the device, it is evident that such a device has unduly high initial cost and is urieconomical in operation.

According to thepresent invention the bridge is directly connected to the high frequency generator and the bridge and the generator are surrounded by a metallic screen or housing which is connected to one of the two points of the bridge between which the measuring instrument is connected, preferably to. that point of the two, which also forms the common connecting point of the two impedances to be compared.- If desired. the low frequency generator, forinstance, an alternating transformer the primary of which is connected to the regular network and the secondary winding of which is connected with the high frequency generator, maybe also surrounded by th screen. I be individual components of my device are also preferably screene from each other, but this is achieved substantially without energy losses.

' The screening ofthe device from the surroundingsis transferred to-the low frequency transformer, whose secondary windingis screened from the surroundings and preferably also from the primary winding and the core of the transformer. As the frequency of the transformer caused by such screening and even if such losses do occasionally occur they do not appreciably increase the costhor cost of operation of the device. According to a further feature of my invention. the parasitic impedances which may occasionally occur in operation, are caused to neutralize each other in a very simple manner.

Further advantages of my invention appear from the following description in which the in, vention is described more fully with reference to some embodiments.v I

I In the drawings forming part of the specification.

' current is low, there are practically no losses,

Figure 1 isa schematic diagram representing 'the well known Wheatstoncv bridge connection used for such measurements.

Figs. 2 to 5fare circuit diagrams illustrating various embodiments of my invention.

Fig. 1 shows a Wheatstone bridge arrangement in which the impedances to be compared Z1 and Connected between the common points A and B of the capacity C1 and impedance Z1 and of the capacity C: and impedance Z: respectively is the high frequency generator E, for instance; a high frequency discharge tube generator or a high frequency dynamo.

Between the common points I) and F of the capacities C1 and C2 and of the impedances Z1.

and Z2 respectively. is connected the measuring instrument V. As a rule, to increase the voltage across the instrument V one or more ampliiying stages are introduced between the points D and F and the meter V.

To obtain a high degree of accuracy, the stray impedances connected in parallel to the arms of the bridge should be equal among themselves, and on-the other hand'coupling impedances between the bridge arms have to beavoided.

For this purpose, the component parts are screened and the screens connected to one point of the bridge.

Also, the influence of the impedances between the points A, B, C and D of the bridge and the surroundings are to be avoided. For this purpose according to the invention the bridge circuits to"- gether with the high frequency generator and the transformer, are surrounded by a metallic housing which is connected to the point F formsing the common point oi. the two impedances Z1 and Z2.

As both or at least one of the impedances Z1 and Z: has to be quickly replaceable, I arrange same outside of the screening housing.

In the arrangements according to the invention the device has an electrically fixed stirrounding and due to the individual screening oi the component parts, such stray or. parasitic innpedances which might occur, can be neutralized in a simple manner.

Figure 2 represents by way of example a for i oi construction according to the invention for The entire device is surrounded by a metallic housing I, indicated in dotted lines, this housing being preferably connected to the point F above Y pedances existing between the points A, B, D, F

and the surroundings, these have to be determined and their effect neutralized.

, The impedance between points D and F, while reducing the sensitiveness of the arrangement do not cause any error in the measurement.

The impedances existing between ,the casing i and the points A and B respectivelyare in parallel 1 with the bridge arms AF and BF respectively,

and should be made equal as far aspossible.

According to the invention it is possible to "use the regular alternating current network for the supply of the measuring current without any disturbing influences.

For this purpose I provide between the net work terminals 22-43 which supply the standard 50 or 60 cycle or 220 volt alternating current and the high Irequency generator E, a transformer T having a primary winding 20 connected across the terminals 22 and 23, and a secondary winding 2i connected to the generator E.

While in the drawingsthe generator E is shown -scl1ematically,-it is to be "understood that the generator may comprise any type oi! high frequency generator which may. be energized by power taken out of the commercial power outlet, as for instance terminals 22 and 213 of. Fig. 2. The energy is fed to the generator E through the transformer T and the output is taken out of the generator E by the other two terminals, as shown. In other words, the low frequency source is utilized to energize the high frequency source,

The windings 2t and ii are wound on the transformer core at a sufiiciently large distance from .each otherfto reduce the intercapscity ei'" feet between each other to a negligible amount.

. from the casing i in'such a. manner as to lteep the dielectric losses low.

'll s also applies to the insulation oi condensers G1 and which furthermore are also to be provided with screens 3th and ti, so as to avoid any change in the. values of the parasitic capacities existing it and B when the capacities ifh and ifh changed.

Furthermore, if very'high irequencies used r'or the testing, the component parts of the bridge are tobe placed on glass insulators so to avoid, losses. For the same reason the inside oi the metallic casings should not be provided with insulating coatings, but left bare.

To eliminate the eflectot. the parasitic impedances I connect variable condensers Cs and C7 between the housing i and the points A and B respectively, by means of which these impedances are caused to neutralize each other.

Figure 3 shows a modification of the device of Fig. 1 which provides a very exact compensation of the capacities. In this figure the showing of the previously described shields has been omitted. ,r' The condenser G1 has a fixed capacity, however, the capacity of condenser C: is adjustable, this condenser in fact consisting oi'three individual capacities C2, C4 and C5. 01' there 03 has a fixed capacity which is smaller than the C1. C4 is a variable capacity the knob or which is accessible from the outside. C5 is a small adlusting condenser, by means of which condenser 'aoaneeo p C1, is so adjusted that in the middle position of condenser C4 the capacities C1 and C: are equal. The capacity of the condenser C4 is a given fraction of capacity C: and changes linearly with also in percentage.

The parasitic capacities between the points.

. A-F and B-F are represented by capacity value 0's and C'-: respectively. If these capacities'are found unequal a corresponding portion of an,

15 adjustable capacity Ca isadded to the side having the smaller capacity value. For instance, if C": is smaller than Us the combined capacity Ce=C"1+Cs should be equal to C's.

To determine the proper adjustments of con- 20 densers Cs and Cs the following procedure is used.

To the points A and B of the device is connected a changeover-switch S, whichin its both positions connects these points to outside terminals X and Y but with reversed polarity. The

'25 switch S is enclosed ina metallic screening casing 40, which is connected to one of the points A or B (in the figureto point'A).

Two condensers C. and Ch, of which one, for

instance Cb, has'to be variable, are now con- 30 nected with one of their electrodes to the points j.

and 40 Suchadjustment is preferably obtained for two values of C. I

45 a value which is large compared to C's (or C'q). Now C5 is adjusted to obtain the above equations while retaining Cs constant, and thereby the values Crand C: are made to closely equaL- For the second adjustment 0 is selected'either sub- 50 stantially equal or smaller than C's (or 0''!) and while Cris held constant, Ca is adjusted to obtain the conditions of the above equations. This adjusts Co to a value close to that of C's (or 0'1).

To, further increase the accuracy of the com- 55 pensation the measurements, and adjustmentsif necessary, are repeated. The condensers C5- and Cs are then fixed at the adjusted values. 1

For the measuring or comparing of the impedances Z1 and Z: these are again connected 60 between the points A-F and b-F respectively,

and the percentual difference between their valuesdetermined with the variable condenser C4. Fig. 4 shows an arrangement which reduces the number of condensers required and simplifies 5 the adjustments. 7

In this case the generator is enclosed'together with the change-over switch-which connects the points A and B to the points P and Q-in a common screening case 10, within which also ,7 falls the secondary winding 2| of the transformer T. The casing H! is connectedat 33 to the point B and is enclosed in a second screening casing which also surrounds the condensers Cl and C: and which is connected to point D.

7 The condenser C: may again consist of three condensers C C4. and Cs as in the arrangement of Fi 3.

The casings I0 and II form a condenser which is connected between the points B and D and thusby suitably dimensioning these casings, the 5 intercapacity of these two casings may replace the fixed condenser Ca. Thus a saving of this capacity is achieved.

The casing l0 together with the voltmeter V, transformer core 34, and primary transformer 10 winding 20, is enclosed in a screening casing I connected to the point F.

From the points P and Q of the switch lead wires 35 and 38 are led to the outside of the casing through suitable apertures. I

To-reduce the capacity between these leads and the point D, and eliminate errors "due to it, when changing the switch. S, from one position to the other, I prefer to provide a further screen I! surrounding the leads 35 and 3i; close to their point of exit from housing H, the screen [2 falling within the screen 1 and being connected therewith.

Bince now all of the component partswhich areconnected to points A and B are completely screened, capacities Cs and C1 become zero and the need for condenser Ca does not exist.

Also the adjustment merely requires the determination of the proper value oLCs.

The screening of the transformer core an of the primary winding 20 is not imperativeas they merely form impedances parallel to the bridge arm D-F which, while causing losses and reducing the sensitivity of the device, do not give. rise to inaccuracies in'the measurement.

Fig. 5 shows a further modification. The generator and preferably also, the changeover switch S, are disposed within a two-part casing l3-M.

The parts l3 and M are similar and insulated and is connected to D. For the first adjustment C is selected to have 'The transformer 34 passes through casing i3-i4 and casing i5 and at this portion is sunrounded by two sleeves, the outside sleeve 26 being connected to casing l5 and the inside sleeve being a two part sleeve 21-28 is connected to the respective portions l3 and ll of the inner casing.

The arrangement is substantially fully symmetrical. a v

To nevertheless take care of any asymmetry I prefer-to provide an adjustable-plate it which A may be connected to one of the'casings. 1

In this arrangement the condensers C1 and Ca may be dispensed with and thus the number of fled anode voltage may be used for the tubes.

While I have described my invention on hand of specific embodiments and in speciflcapplica- .tions. 1 do not wish to be limited to same, but desire the appended claims to be construed as broadlyas possiblei'n view of the prior art. In the claims the term comparing of impedance's" also' includes the measuring of impedances.

What I claim is: i 1. A device for comparing impedances, comprising abridge arrangement, the-four branches of which comprise two known impedances and two impedances to be compared. a high frequency generator for feeding the bridge and a two-part screening casing surrounding the genfrom each other, one of said halves being connected toone terminal of the generator and the other half being connected to the other terminal of the generator. I 1

2. A device for comparing impedan'ces, com-- prising a bridge arrangement, the i'ourvbranches of which comprise two known impedances and two impedances to be compared, a high frequency 7 generator for feeding the bridge and a two-part screening casing surrounding the generator and having symmetrical halves insulated from each other; one of said halves being connected to one terminal of the generator and the other half be ing connected to the other terminal of the generator, and a low frequency transformer having a primary winding, a secondarywinding and a core, said secondary winding being associated with the high frequency generator to energize the same, and apertures through said casing through which said core passes, said secondary Winding falling within said screening casing and a screem ing casing enclosing both the transformer 58C" ondary and the high frequency generator.

-9,081,60 *erator and having symmetrical halves insulated 3. A device for comparing impedances com- .prising a bridge arrangement provided with four branches or which two comprise known impedances and two comprise impedances to be compared, a high frequency generator connected across opposite points of the bridge, a floating shield surrounding the generator, said excitation points of the bridge leading into the floating shield, an indicator device connected across the other two opposite points 01 the bridge, and

' means for connecting one end of the indicator device to the floating shield.

4. The device described in the next preceding claim-further characterized by that there pro- 

